Hydrocarbon resources such as petroleum or natural gas have come to be produced by excavation through wells (oil wells or gas wells, also collectively called “wells”) having a porous and permeable subterranean formation. As energy consumption increases, deeper wells are being drilled, reaching depths greater than 9000 m worldwide and greater than 6000 m in Japan. In wells that are continuously excavated, the productive layer is stimulated in order to continuously excavate hydrocarbon resources efficiently from subterranean formations of which permeability has decreased over time and subterranean formations of which permeability has gradually become insufficient. Known stimulation methods include acid treatment and fracturing. Acid treatment is a method in which the permeability of the productive layer is increased by injecting a mixture of strong acids such as hydrochloric acid and hydrogen fluoride into the productive layer and dissolving the reaction components of bedrock (carbonates, clay minerals, silicates, and the like). However, various problems that accompany the use of strong acids have been identified, and increased costs, including various countermeasures, have also been pointed out. Thus, methods for forming fractures (also called “fracturing” or “hydraulic fracturing”) in the productive layer using fluid pressure have received attention.
Hydraulic fracturing is a method in which fractures are generated in the productive layer by fluid pressure such as water pressure (also simply called “hydraulic pressure” hereinafter). Generally, a vertical hole is drilled, and then the vertical hole is curved and a horizontal hole is drilled in a subterranean formation several thousand meters underground. Fracturing fluid is then fed into these boreholes (meaning holes provided for forming a well, also called “downholes”) at high pressure, and fractures and the like are produced by the hydraulic pressure in the deep subterranean productive layer (layer that produces the hydrocarbon resource such as petroleum or natural gas), and the productive layer is thereby stimulated in order to extract and recover the hydrocarbon resource through the fractures. The efficacy of hydraulic fracturing has also been examined for the development of unconventional resources such as shale oil (oil that matures in shale) and shale gas.
The following method is typically used to produce fractures and to perform the well treatment of perforation by hydraulic pressure in the productive layer of a deep subterranean formation (layer that produces the hydrocarbon resource such as petroleum such as shale oil or natural gas such as shale gas) using high-pressure fluid such as fracturing fluid. Specifically, a prescribed section of a borehole (downhole) drilled in a subterranean formation several thousand meters deep is partially plugged while isolating (also called “sealing”) sequentially from the tip portion of the borehole, and fractures are produced or perforation is performed in the productive layer by feeding a fluid such as fracturing fluid at high pressure into the plugged section or using a tool containing an explosive compound such as a perforation gun. Then, the next prescribed section (typically ahead of the preceding section, i.e., a segment closer to the ground surface) is plugged, and fracturing and the like are performed, causing the fractures and perforations to advance. After that, this process is repeated until the required isolation, fracturing, and the like have been completed.
Stimulation of the productive layer is sometimes also performed again by fracturing not only for drilling of new wells but for desired sections of boreholes that have already been formed. In this case as well, the operations of borehole plugging, fracturing, and the like are similarly repeated. Additionally, there are also cases where, to perform finishing of the well, the borehole is plugged to block fluid from below, and after finishing of the top portions thereof is performed, the plugging is released.
A variety of downhole tools, which are tools used in a well to perform plugging and fracturing of a borehole, are known. For example, Patent Documents 1 to 3 disclose plugs (also called “frac plugs”, “bridge plugs”, “packers”, and the like) which plug or fix a borehole by various members (various elements) being disposed on the periphery of a core metal.
Patent Document 1 discloses a degradable bridge plug in which a slip made of metal or a seal made of an elastic material is disposed on the outer circumferential surface of a mandrel. Patent Document 2 discloses a downhole tool such as a frac plug, bridge plug, or packer comprising a slip, sealing element, or the like on the outer circumferential surface of a body member, and further comprising a ball which is an unidirectional valve that prevents fluid from flowing through, and also discloses that the tool is degraded by a chemical solution. Patent Document 3 discloses a sleeve system (also called a “frac sleeve”) in which fracture sleeve pistons, in which a passageway penetrates through the center part, are sequentially arranged so as to be movable in the axial direction of the sleeve, and sequential closed spaces are formed by ball sealers (also simply called “balls”) and ball valve seats (also called “ball seats” or simply “seats”).
Downhole tools used for well drilling are sequentially disposed in the borehole until the well is completed and need to have sealing performance such that they plug (seal) the prescribed sections inside the borehole against the fluid pressure for performing well treatments such as fracturing and perforation by high-pressure fluid. At the same time, the seal needs to be easy to release when the prescribed well treatment is completed and the subsequent well treatment is to be performed. Additionally, the seal needs to be released and the used downhole tools removed at the stage when production of petroleum such as shale oil or natural gas such as shale gas (hereinafter also collectively called “petroleum and natural gas” or “petroleum or natural gas”) or the like is begun. Because a downhole tool such as a plug is typically not designed to be retrievable after use and release of plugging, it is removed by destruction or by making it into small fragments by milling, drill out, or another method, but substantial cost and time are required for milling, drill out, and the like. There are also plugs specially designed to be retrievable after use (retrievable plugs), but since plugs are placed deep underground, substantial cost and time are required to retrieve all of them.
Patent Document 1 discloses comprising a degradable member; and discloses being formed from a reactive metal selected from aluminum, calcium, and magnesium or an alloy of lithium, gallium, indium, or the like; and also discloses that a seal is formed by an elastic material capable of drill out. Patent Document 2 discloses a material composed of an epoxy, glass fibers, and the like as a tool degraded by a chemical solution. Additionally, Patent Document 4 discloses that a degradable polymer, such as aliphatic polyester, polycarbonate, or polyamino acid, in a downhole tool such as a plug or sleeve is degraded by a basic solution. Note that Patent Document 3 does not disclose that the frac sleeve is degradable.
Due to increased demand for securement of energy resources and environmental protection, particularly as excavation of unconventional resources expands, on the one hand, excavation conditions are becoming increasingly harsh, such as increased depth, while on the other hand, excavation is advancing under a diversity of excavation conditions, for example, a diversity of environmental conditions such as from temperatures lower than 60° C., e.g. 25° C., to high temperatures of approximately 200° C. attendant to the diversification of depth.
Specifically, downhole tools such as frac plugs, bridge plugs, packers, cement retainers, and sleeve systems (frac sleeves) need to have, on the one hand, mechanical strength (tensile strength and compression strength) to allow the material to be transported to a depth of several thousand meters underground, as well as oil resistance, water resistance, and heat resistance such that mechanical strength and the like are maintained even when they come in contact with the hydrocarbon resource to be recovered in the high-temperature and high-humidity environment of a deep subterranean downhole. Furthermore, downhole tool members provided in downhole tools need to have various characteristics such as sealing performance such that they can maintain plugging even against high-pressure hydraulic pressure by sealing fluid between the downhole tool and the inner wall of the borehole, specifically the casing disposed inside the borehole, when plugging the prescribed space of the downhole for performing perforation or fracturing.
At the same time, downhole tools and/or downhole tool members need to have the characteristic of being able to release the seal as necessary. Additionally, downhole tools and/or downhole tool members need to have the characteristics of being both easily removable and capable of improving production efficiency by completely releasing the fluid seal within a desired period under the environmental conditions of the well at the stage when the well for hydrocarbon resource recovery is completed (as described above, there are a diversity of environments such as temperature conditions attendant to diversification of depth). Thus, seal members for downhole tools directly involved in fluid sealing have also come to be designed in consideration of sealing performance and convenience of seal release at the same time.
That is, due to the fact that excavation conditions have become diverse such as increased depth, there is a need for a well treatment method that can decrease expense and shorten processes of well drilling by enabling design of the acceleration of release of a seal by a seal member for downhole tools as desire and the removal of downhole tools and the securement of flow paths as desire, which are performed in well treatment.